Amniotic Egg in Reptiles and Mammals

Amniotes are tetrapods with special characteristics that enable them to produce an amniotic egg. In the majority of species this egg is adapted to be laid on land and survive. However there are mammals, such as eutherian mammals that utilize a similar structure for reproduction.

An amniotic egg is made up of an amnion, which is a fluid-filled sac that protects the embryo. This type of egg also has membranes that develop outward from the embryo and are not part of it. Extra-embryonic membranes include an amnion, chorion, yolk sac, and an allantois.

The amnion embraces the embryo and proportions it with an aqueous habitat, known as the amniotic fluid. While amphibians have the necessity to lay their eggs in water, the amniotic egg enables the animal to place its egg on land because it contains the water inside of it.

Attached to the embryo are the allantois and the yolk sac. The yolk sac, which is concentrated with nutrients, feeds the embryo as it grows. As the embryo matures, the yolk sac reduces. All of the embryo´s metabolic wastes are stored in the allantois. This last membrane also functions as a gas exchanger along with the chorion. The overall enclosure of the membranes and embryo is provided by the chorion. It is in charge of exchanging gases, retaining moisture, and diffusing carbon and oxygen across the membrane.

Finally, there is the albumen or outer shell of the egg. This one protects the whole egg and is around the chorion. The shell prevents dry circumstances for the embryo, while still permitting the creature to receive air.

Mammals use a type of modified egg structure, normally within their uterus called placenta. The young of these mammals are developed inside the mother from fertilization, when the sperm of the male joins with the ovum, or “egg” of the female. Placental mammals are viviparous, which means that they give birth to fully developed offspring that was carried by the mother through its growth.

The membranes in placental mammals are like the ones of the laid egg, but modified in a certain manner. Surrounding the embryo is the amnion filled with the amniotic fluid. The yolk sac and the allantois are joined to become the umbilical chord, where food is supplied to the fetus and the waste of it is also removed. All of these membranes, including part of the chorion, compose the placenta. This one in turn physically attaches the embryo to uterine wall of its mother. The placenta also protects the embryo and the mother by separating their blood.


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